Patent GB-1330490-A (GB)
Title: Semiconductor controlled rectifier switching devices
IPC: H01L27/08
Filed: 1970-11-05
Abstract: 1330490 Semi-conductor devices WESTING- HOUSE ELECTRIC CORP 5 Nov 1970 [10 Nov 1969] 52627/70 Heading H1K In a semi-conductive controlled rectifier switch element 10 of cascaded structure a body 12 of Si (or alternatively Si carbide, Ge, or IIb VIIb or IIIb Vb compounds) regions 18, 20, 22 have 1st type conductivity and regions 24, 26, 28, 30 have 2nd opposite type conductivity to form PN-junctions 32, 34, 36, 38; the element having PNPN or NPNP configuration of all diffused or diffused/alloyed structure. Regions 22, 24, 26 are formed by double P-type diffusion into N-type substrate, and regions 18, 20, 28, 30 by recrystallization of region 24 when metal contacts 40, 42, 44, 46 are alloyed to the regional surfaces. Contacts 40, 46 have external leads 48, 50 and 42, 44 are interconnected by lead 52 to short out PN-junction 34. The undersurface of the body has a recrystallized P-type region 56 alloyed to a support 54 of, e.g. Mo, Wo, Ta or  alloys thereof to provide ohmic contact to region 26. In operation a small gate current through contact 46 and gate 30 of a subsidiary rectifier switch controls a larger gate current in a main rectifier switch 26, 22, 24, 18 over gate 28; the rectifier switches being integral within a single s./c. body with the main switch surrounding the subsidiary, so that the system operates quickly and withstands a high rate of current rise. A pulse IG 1  to contact 46 turns on a smaller central switch comprising regions 26, 56, 22, 24, 30 and 20 and the interregional junctions (Fig. 3). A larger switch surrounding the smaller comprises region 26 and crystallized region 56, regions 22, 24, recrystallized regions 28, and 18 and the inter-regional junctions; with its second gate comprising regions 24 and region 28 beneath contact 44; the cathode current IK 1  of the first switch furnishing the gate current IG 2  of the second switch over the effective resistance of region 24 to control cathode current IK 2  = anode current IA. The second gate has a large area compared with the first; minimizing turn on dissipation and potential burn out. In a modification (Fig. 4) a semi-conductor element 110 fabricated by diffusion or epitaxial growth followed by diffusion comprises a body 112 of s./c. material formed with a first emitter or cathode region 114, a first base or gate 116, a second base or gate 118, a second emitter 120 and a third emitter 112; PN-junctions 124, 126, 128, 130 separating the regions of opposed conductivity type 114, 116; 116, 118; 118, 120 and 122, 116, to form a PNPN or NPNP configuration; the regions 114, 122 being of similar semi-conductivity. The surfaces of emitter regions 114, 122 and gate region 116 lie in the upper surface of element 110, and cathodic ohmic contacts 134 of metal, e.g. Al, Au, Ti, Ag overlie a portion of 114, while a gate contact 136 of similar metal overlies region 16 and third contacts 138 of similar metal overlap regions 122, 116 and covers the exposed portion of the PN-junction 130. Contact 136 may be symmetrial of surrounding contacts 138 lying within contacts 134 and functions similarly to contacts 42, 44 (Fig. 1). A support member 140 is ohmically soldered to the lower surface 142 of the body, and the initial switch comprises regions 120, 118, 116, 122 with contact 136 operating as first gate to region 116. The cathode current IK 1  of the initial switch is gate current for the second switch comprising regions 120, 118, 116, 114 with contact 138 operating as cathode of both the first and the second switches with contact 136 operation also as second gate. An external resistance may be added in connection 52 to reduce internal heating and inverse switching speed, to stabilize operation, and improve the tolerance of high current rate of change. In a modification (Figs. 5, 6, not shown) the surface regions are concentrically disposed of the first gate contact with. multiple radial secondary gates and ohmic bridging connections between a cathode region of the first switch and a gate region of the second switch; the surface being coated with insulant, e.g. SiO 2  or Si 3 N 4  or a composite thereof to prevent short circuiting to the underlying regions. These bridge connections may be resistive, and the device may be of planar or mesa form. The planar or mesa element may have duplicate continuous or arcuate gate and cathode regions (Figs. 7, 8, not shown). In a further modification (Figs. 8, 10) the resistive arcuate or annular bridging member may be of nichrome or conductive ceramic, and the outer first conductivity type region is resiliently contacted by a large area copper electrode centrally apertured to admit an insulated jacketed first gate connection resiliently held against the central first gate contact; the bridging contact being held in an axial recess of the copper contact and insulated therefrom by a recessed synthetic resin and/or fibre glass insulant body. The element may be encapsulated with resilient support, and the second gate and second cathode regions may be interdigitated. In fabrication, a silicon N-type wafer is lapped, polished, and diffused through both surfaces with Al and Ga to produce a N-type region sandwiched between spaced P-type regions to form contiguous PN-junctions, and diffused with P to surround it with a N-type region, which is removed by selective photolitho etching from the sides, one surface, and selected parts of the other surface to produce a mesa with annular projecting N-type regions surrounding a central P-type region and in turn spacedly surrounded by an annular N-type region containing four spaced surface exposed P-type regions. Ohmic contacts are attached to the exposed regional surfaces, and a Mo electrode soldered to the remaining surface, while leads are fixed to the central P-type region, to the surrounding N-type regions, and to the intercalated P-type exposed regions.